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post #1 of 15 (permalink) Old 09-07-2019, 12:52 AM Thread Starter
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I bought my vn750 as a non runner a few months ago after owning Vulcan 1500 and EN500 which both run great. After many attempts found and changed a faulty coil cleaned carbs repaired sticking floats cleaned tank and put in new fuel also did an earshave for an ease of carb removal. It all appeard good ticking over and at low speeds under 40 and at full throttle but cruising was a nightmare with holding back and misfire? I've tried multiple jettings cleaned and checked carbs many times tried different carb vent locations checked resistance of pickups and coils new plugs leads etc but it still runs badly. Bike charging and voltage is fine.i moved pickups nearer stator as in forum but my bad cruising still persists. I keep ending up looking at the CDI box and In the end checked resistance in it all seems well except for resistances to pin 2- seem to be double. I opened it up and visually it looks ok. Any one out there got any answers to my cruising 1/4 throttle bad running
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post #2 of 15 (permalink) Old 09-07-2019, 11:18 AM
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Responded to your post in Newbie Introductions before seeing this.
What jets are you running? You're saying it idles fine, and runs fine at high rpms (5500 + up) but will not run smooth in between?
Seems likely still a carb problem. I'd make sure the rubber diaphragms in each carb are opening evenly at those engine speeds. (A tiny hole or tear in one could be the problem)
Carb needle positions meters fuel at those speeds. Perhaps ones loose?

I'll let someone else answer the electrical questions here....

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post #3 of 15 (permalink) Old 09-07-2019, 03:52 PM Thread Starter
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140 - 42 jets took shims out(2) but have tried different jets and shim combinations don't make any difference. Have checked diaphragms several times. It sometimes only starts on one cylinder for perhaps 1-2 seconds sometimes and when you rev it up at standstill does not feel as crisp as it should.plugs are a nice color. It does not use water . I run bikes on common land in my younger days with no silencers or air filters and they ran fine without touching jets in carbs. My mechanic friend thinks it's electrical as well as myself. There are no CDI boxes for sale in UK at moment and to get one from usa is big money especially if it's not that playing up.
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post #4 of 15 (permalink) Old 09-07-2019, 05:53 PM
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I see you ear shaved, where is the bowl vent line routed?? it is critical to NOT be in turbulent air. a good spot is up under the tank, either attached to the empty nipple on the fuel sender cover, or secured to the TOP of one of the frame rails

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post #5 of 15 (permalink) Old 09-08-2019, 04:47 AM
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It is unlikely that the CDI is the problem. All anecdotal evidence indicates that they fail completely when things go wrong. Pin 2 is something of a mystery, but speculation is that it changes the timing ever so slightly when starting the bike, but it "should" have no effect once the bike is running, so your resistance checks on all the other pins would indicate that the CDI wouldn't be the problematic component.

You could probably rig up something to check the consistency of your spark on each cylinder, but I'm not sure how visually accurate something like a timing light would be if you think it's just missing every couple sparks. Something like an oscilloscope would for sure do the trick, but that's not something you find under most workbenches!

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post #6 of 15 (permalink) Old 09-08-2019, 09:17 AM
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Quote:
Originally Posted by Thorn View Post
It is unlikely that the CDI is the problem. All anecdotal evidence indicates that they fail completely when things go wrong. Pin 2 is something of a mystery, but speculation is that it changes the timing ever so slightly when starting the bike, but it "should" have no effect once the bike is running, so your resistance checks on all the other pins would indicate that the CDI wouldn't be the problematic component.

You could probably rig up something to check the consistency of your spark on each cylinder, but I'm not sure how visually accurate something like a timing light would be if you think it's just missing every couple sparks. Something like an oscilloscope would for sure do the trick, but that's not something you find under most workbenches!
That’s interesting, would the standard Oscope probe end handle that spark? I have a scope from when I graduated electronics tech school back in the mid-90’s. The school was refreshing their equipment so they sold off their existing stuff. I landed a job/career in telecom engineering so the need for component level testing went away long long ago and it’s been shelf bound ever since.
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post #7 of 15 (permalink) Old 09-08-2019, 10:28 AM Thread Starter
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I tried some testers that went between the plugs and caps and the spark looked ok. I've worked on cars and bikes all my life and changing mechanical bits . but finding the faults can be tricky.ive tried venting carbs in what I think is sheltered area Inc plastic tank gauge nipple and nothing has made any difference.
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post #8 of 15 (permalink) Old 09-08-2019, 10:29 AM Thread Starter
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If you take CDI apart pin 2 seems to go nowhere
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post #9 of 15 (permalink) Old 09-08-2019, 10:39 AM
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Quote:
Originally Posted by mmart View Post
Thatís interesting, would the standard Oscope probe end handle that spark?
I don't know the numbers, but the primary coils of the ignition coil is just at 12v nominal. You wouldn't want to hook a scope up to to the spark plug leads.

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post #10 of 15 (permalink) Old 09-08-2019, 10:52 AM
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Originally Posted by snowy View Post
If you take CDI apart pin 2 seems to go nowhere
But you're getting a non-infinite resistance value between pin 2 and pins 3, 5, and 6, right? It's got to connect to SOMETHING?

If you still have the CDI box open and around, some good quality pictures of both sides of that board would be amazing to have posted here.

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